1. Field of the Invention
The ability to introduce a chiral center stereospecifically or resolve a racemic mixture has broad industrial applications. Natural products having chiral centers are normally stereoisomeric, with only one of the stereoisomers being physiologically active. Where synthesizing or modifying a natural product involving the formation of a stereoisomeric center, the ability to introduce the functionality asymmetrically is of extreme economic importance. Where only one enantiometer is active, not only is half of a racemic product inactive, but where it is required to separate the inactive product, the recovery of the desired stereoisomer results in further losses of the desired product. In synthetic procedures involving a number of steps, an intermediate or final step resulting in a yield less than 50% will seriously affect the economics of the synthetic approach.
Furthermore, with polymers, stereoregularity can provide for greatly enhanced physical properties. Because asymmetric compounds other than naturally occurring compounds are frequently costly, stereoregular polymers of asymmetric monomers have not found extensive use.
There has been, therefore, a continuing interest in being able to prepare compounds by asymmetric induction to provide a product having enhanced amounts of a particular stereoisomer.
2. Description of the Prior Art
Methods of asymmetric synthesis may be found in J. D. Morrison and H. S. Mosher, "Asymmetric Organic Reactions," Prentice-Hall, Englewood Cliffs, N.J., 1971 258-62; S. Yamada et al. J. Am. Chem. Soc., 99, 1988 (1977); R. C. Michaelson et al. ibid., 99 1990 (1977); H. B. Kagen et al. Angew. Chem. Int. Ed. Eng., 18 45 (1979) K. Tani et al., Tetrahedron Lett., 3017 (1979); and H. Wynberg and B. Marsman, J. Org. Chem., 45 158 (1980); K. B. Sharpless and T. R. Verhoeven, Aldrichimica Acta, 12 63 (1979). Tani et al., Tetra. Letters 32, 3017-3020 (1979) describe the use of molybdenum catalysts to symmetrically epoxidize olefinic hydrocarbons in the presence of chiral diols.